Bituminous compositions



United States Patent 3,070,523 BITUMINOUS COMPOSITIONS Stephen H. Alexander and Gilbert W. Tarver, El Dorado, Ark., assignors to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed June 22, 1959, Ser. No. 821,636 2 Claims. (Cl. 204-147) This invention relates to bituminous compositions and more particularly to improved asphaltic coating compositions and the application of such compositions to metallic surfaces.

Cathodic protection is a widely used method for decreasing the rate of corrosion of metallic surfaces which are exposed to electrolytes. Cathodic protection is well described in the art and comprises the use of an impressed current to prevent or reduce the rate of corrosion of a metal in an electrolyte by making the metal the cathode for the impressed current.

While cathodic protection can be and is employed on a wide variety of metallic surfaces or structures, it is particularly well suited for the protection of metal pipes installed or buried beneath the surface of the earth. A heavy current demand results when cathodic protection is employed on bare metal surfaces. Therefore, as a general rule, underground pipes and other metallic surfaces are protected by both a protective coating and by cathodic protection. Bituminous compositions, of either coal tar or petroleum origin, have become widely used as protective coatings for metallic surfaces. It has been found, however, that such coating compositions have a tendency to disbond from the metal surface when the coated metal surfaces are subjected to cathodic protection. This disbonding is emphasized, seriously so, in the areas of any holidays-or breaks in the protective coating.

It is an object of this invention to provide an improved bituminous coating for metallic surfaces which are to be subjected to cathodic protection. It is also an object of this invention to provide an improved process for protecting metal surfaces which are to be subjected to cathodic protection. Other objects will become apparent from the description of this invention;

When using bituminous protective coatings which are designed for application at elevated temperatures, it is necessary that the metallic surface first be given a prime coat. This is generally a cutback bitumen applied at ambient temperatures. After the prime coat has set, the finish coat is applied, generally at elevated temperatures. It has now been discovered that the disbonding of a bituminous coating composition from a metal surface under cathodic protection can be either prevented or significantly retarded by incorporating into the cutback bitumen prime coat -a minor amount of a collidine. According to a preferred embodiment of this invention, an improved asphaltic prime coat for metallic surfaces is provided comprising a liquid hydrocarbon vehicle, an asphalt and a minor amount of a collidine as hereinbefore described. Substantially improved protection of metallic surfaces subjected to cathodic protection is achieved by applying to said metallic surface a prime coat as above described and then a finish asphalt coat of the type well known to those skilled in the art.

To illustrate this invention, a cutback asphalt primer was prepared using 40% by weight of an airblown petroleum asphalt having a softening point of 195 F. and a ice penetration of 7 mm./10 at 77 F. and 60% by weight of petroleum naphtha having a boiling range of 300 to 360 F. The primer had a viscosity of 40 Saybolt Furol seconds at 77 F.

The finish asphalt coating was prepared by mixing from 19 to 21% by weight of total composition of slate flour having a fineness such that at least passed through a 325 mesh sieve with an airblown asphalt having a softening point of 228 to 234 F. and a penetration of 7 to 9 mm./ 10 at 77 F. The finish coat had a viscosity of approximately 900 Saybolt Furol seconds at 375 F.

A clean set of metal panels were then brush coated with the primer as above described. Another set of clean metal panels were brush coated with the primer as above described to which had been added 1% by weight of total composition of a mixture of isomeric collidines. The panels were allowed to dry overnight and then a 55-65 mil coating of the finish coat as above described was applied at 450 to 460 F. using a plaqueing machine.

After the panels were dry, an intentional holiday (l/16) was punched into the surface of each coated panel and the panel immersed in an electrolyte having the following composition:

Compound: G/l. of solution Magnesium chloride hexahydrate 11.0

Calcium chloride 1.2 Sodium sulfate 4.0 Sodium chloride 25.0

The specimens were placed under cathodic protection with sufficient current being drained to the specimens to maintain a saturated calomel potential of -1.5 volts. After seven days the specimens were removed and examined for disbonding. Disbonding was significantly re duced on those panels which had first been primed using the novel prime coating of this invention.

The origin of the bitumen used in preparing the novel prime coat of this invention is not a critical factor. It may be coal tar or petroleum asphalt. .Preferred prime coats of this invention utilize a petroleum asphalt having a softening point of F. to 250 F. and a penetration of 0 to 25 mm./1O at 77 F. The bitumen content of the prime coat can vary but preferably it is maintained in the range of 35% to 55% by weight of total composition. The liquid hydrocarbon vehicle can also vary in composition and conventional vehicles well known to those skilled in the art can be used. Hydrocarbons boiling in the range of 200 to 400 F. are particularly preferred.

Any of the isomeric collidines can be used to prepare the prime coats of this invention. Collidines which are useful and can be used include the 4-ethyl-2-methylpyridine, 3-ethyl-4-methylpyridine and 2,4,6-trimethylpyridine. Only a minor amount of this additive is required to realize the benefits of this invention. Preferred prime coats of this invention contain from about 0.05 to 5.0% by weight of total composition of the additive.

Any conventional bituminous finish coat can be used with the novel primer of this invention. Again, the bitumen can be coal tar or petroleum asphalt. The finish coat can contain various fillers and additives such as are 3, usually found in the generally used coating compositions.

What is claimed is:

1. A method of protecting metallic surfaces against disbonding of coatings thereon, said method consisting essentially of (1) coating said metallic surfaces with an improved prime coating composition consisting essentially of a bitumen in an amount ranging from about 35% to about 55% by weight of total composition of said prime coating, a liquid hydrocarbon vehicle and from 0.05% to 5.0% by weight of total prime coating composition of a collidine, (2) thereafter coating said metallie surfaces having the prime coating composition adhered thereto with a finish bitumen coating and (3) subjecting the coated metallic surfaces to cathodic protection.

2. The method of claim 1 wherein said bitumen is a petroleum asphalt having a softening point of 150 F. to 250 F. and a penetration of 0 to 25 mm./10 at 77 F.

References Cited in the tile of this patent UNITED STATES PATENTS 2,685,523 Cross Aug. 3, 1954 

1. A METHOD OF PROTECTING METALLIC SURFACES AGAINST DISBONDING OF COATINGS THEREON, SAID METHOD CONSISTING ESSENTIALLY OF (1) COATING SAID METALLIC SURFACES WITH AN IMPROVED PRIME COATING COMPOSITION CONSISTING ESSENTIALLY OF A BITUMEN IN AN AMOUNT RANGING FROM ABOUT 35% TO ABOUT 55% BY WEIGHT OF TOTAL COMPOSITION OF SAID PRIME COATING, A LIQUID HYDROCARBON VEHICLE AND FROM 0.05% TO 5.0% BY WEIGHT OF TOTAL PRIME COATING COMPOSITION OF A COLLIDINE, (2) THEREAFTER COATING SAID METALLIC SURFACES HAVING THE PRIME COATING COMPOSITION ADHERED THERETO WITH A FINISH BITUMEN COATING AND (3) SUBJECTING THE COATED METALLIC SURFACES TO CATHODIC PROTECTION. 